1. Field of the Invention
This invention relates to frequency modulation (FM) communication systems. More particularly, this invention relates to a threshold extension demodulator for an FM system.
2. Description of Related Art
Frequency modulation is commonly used as a method for transmission of multi-channel telephone information. A representative system may have a designated capacity of 900 separate channels. At any given time, only a portion of these channels are in use. As requirements change, the system may expand or contract in certain predetermined incremental steps. For example, there can be increments of 300 channels each. Thus, the system may configure itself to handle 300, 600, or a maximum of 900 separate channels.
The loading requirements of the system may change due to various factors. One of these, of course, is traffic demands. During the middle of the day, for example, a maximum number of conversations are occurring, requiring greater loading. Loading requirements may also vary with the seasons. In the summer, especially during rainy periods, a larger number of channels are required due to individual channel failure.
Any given system has an optimum bandwidth. The amount of loading dictates the actual bandwidth in use. It is not possible, given current technology, to design a system which will accommodate a change of bandwidth of one to three, which would correspond to the loading variations discussed above. Further, for any given bandwidth, the system has a fixed quantity of inherent noise. If the system demodulator is optimized for a particular bandwidth, the noise characteristic will remain constant, regardless of the bandwidth of the baseband signal present. When the carrier power in the bandwidth approaches the noise level of the receiver, a sudden and substantial degradation will occur in the voice channel performance.
This phenomenon, present in all FM systems, is commonly known as the FM threshold effect, which occurs when the received carrier level decreases to the point where it approaches the thermal noise level of the receiver. When conventional wide band FM demodulators are used, this FM threshold occurs when the carrier is 10 to 12 dB above the thermal noise level of the receiver. As the carrier level decreases below this point, a substantial increase in background noise occurs in the telephone channels and bit-error-rates (BER) increase significantly for data communication circuits within the telephone channel.
A threshold extension demodulator can decrease the point at which FM threshold occurs by up to 7 dB under some conditions. That is, the point at which the FM threshold occurs is 3 to 5 dB above the thermal noise level of the receiver. A threshold extender therefore increases the dynamic range of an FM receiver by its threshold improvement factor, 7 dB in the above example. This phenomenon is well understood and the use of threshold extension techniques is quite common these days. Several types of threshold extension demodulators are in use and some of them are known by the following terminology: the phase-locked loop demodulator, the FM feedback demodulator, and the tracking filter demodulator.
Threshold extension demodulators are normally designed to compensate for one specific bandwidth. In a system where the bandwidth is subject to constant change, the benefit gained by the extension demodulator will be lost as the threshold point will not be optimized for the different bandwidths encountered by the demodulator.